Summary Multiple co-occurring chronic diseases (multimorbidity) is highly prevalent among older adults, especially females, and often require simultaneous use of multiple medications (polypharmacy). Polypharmacy increases risk for potentially inappropriate prescribing (PIP), ED visits and hospitalization. Existing gender differences in prescription patterns can lead to different health outcomes in men and women with multimorbidity. There is limited understanding of the effect of the complexity of chronic condition patterns on treatment and healthcare outcomes, and the role of gender in this process needs further evaluation. Sex-related preclinical research in pharmacology can only partially explain role of gender, further exploration of clinical and social factors is needed. The proposed study will add to our understanding of the gender differences in polypharmacy volume among older adults with multimorbidity and help to identify how PIP varies by gender across multimorbidity patterns. In addition, evaluating the gender differences in medication-related ED visits/hospitalization among adults who experience PIP provide actionable knowledge related to drug use across various multimorbidity patterns. The proposed cross-sectional study will utilize NIA-funded Health and Retirement Study (HRS) data with additional linkage to Medicare and supplemental Prescription Drug Study datasets. This comprehensive dataset enables to consider wide range of socioeconomic, clinic and utilization factors in our study. We propose two aims for the supplement: Aim 1: Determine gender differences in polypharmacy levels across various multimorbidity patterns among older adults. Aim 2: Evaluate gender differences in the association between PIP and medication-related ED visits/hospitalization among older adults with various multimorbidity patterns.